The field of the invention is the field of optical lenses, and in particular optical lenses for use in photolithography in the semiconductor industry.
Advances in computers have largely been driven by advances in photolithography, where better lenses and shorter wavelength light allow exposure of ever finer features. In large part, the lens design process is also driven by the advances in computers, as it becomes ever faster to trace rays and numerically optimize lens design. A great deal of scope for the experienced and talented lens designer still remains, however, as the computer does not yet xe2x80x9cknowxe2x80x9d which of the interrelated variables to optimize first, nor which variable is less or more sensitive to variations in manufacture or adjustment.
In particular, as the photolithographic wavelength shrinks from around 250 nanometers (nm) to 190 nm and to 157 nm to take advantage of available laser sources of light, the number of adjustable parameters available to lens designers shrinks even faster. In the visible and near ultraviolet region, there are a multitude of glasses which have the required homogeneity and transparency and which can provide a range of index of refraction and dispersion coefficients needed for production high numerical aperture (N. A.) diffraction limited lenses. Fused silica, the material of choice for lenses, starts to absorb at 160.5 nm, and at 157 nm the absorption in the lens material would preclude use. The only practical material for the construction of refractive lenses for the shorter wavelengths is a fluorite material, and in particular calcium fluoride.
Refractive lenses limited to only one material need a very large number of elements, (above 35 in some cases for flat field lenses having a large image size and high N. A.), and need a very large diameter. The cost of such lenses increases at least as the 3.8 power of the diameter, and the material cost and the fabrication costs become prohibitive for lenses with many elements and large dimensions.
Such lenses are now widely used in photographic and projecting equipment, television cameras, microscopy, and as of late, manufacturing equipment in the semiconductor industry. Here, anastigmatic lenses are used in step-and-repeat or scanning cameras for patterning microprocessors, memory and logic chips, etc. These optical systems can be divided into two classes. The most common class is the extension of the double Gauss lens due to Glatzel (Zeiss Company) reported at the International Lens Design Conference, Mills College, Calif. in 1981, and published by SPIE and OSA. The Glatzel lens exhibits a xe2x80x9cdouble bulgexe2x80x9d. In the Glatzel designs, the field curvature is corrected by two or more shrinks and expansions of the bundle of rays passing through the lens. This compares with just a single shrink in the conventional photographic lens, such as a triplet or a double Gauss lens. Such lenses are extremely expensive because many more lens elements are used. The other class of lenses used are the xe2x80x9cring systemsxe2x80x9d, where the optics is corrected along an annulus and no effort is made to correct the field curvature. Ring system lenses are inefficient in their use of the light. Ring systems have an advantage over symmetric lens systems that light does not propagate through the entire volume xe2x80x9cseenxe2x80x9d by the lens elements, and optical elements such as""turning mirrors may be placed on or near the optical axis to act as turning mirrors. Reflective lens elements are inherently achromatic, and may be used at the shortest wavelengths. However, the aberrations introduced by reflection from curved surfaces at non normal incidence have convinced many designers skilled in the art that such mirrors can not give diffraction limited performance at high numerical apertures in an axially symmetric system.
One telescope designer, D. D. Maksutov, published a design of an aplanatic telescope in a book called ASTRONOMICZESKAYA OPTIKA, published by OGIZ (Leningrad) in 1945, using two curved mirrors. This design is a version of the Gregory telescope where the secondary mirror is also concave, and is positioned beyond the focal point of the primary mirror. By definition, an aplanatic telescope must have a correction for spherical aberration and coma. The two simultaneous quadratic equations which must be solved have real roots only for a rather large central obstruction and a real image plane in front of the primary mirror. The telescope as designed left all aberrations and their higher order residuals uncorrected except for Seidel type spherical aberrations and coma, and there is no way of correcting them further.
Useful references for Lens Design are: A. E. Conrady, Applied Optics and Optical Designs, 2nd Edition, Dover Publications, in 2 volumes 1957 and 1960; H. H. Hopkins, Wave Theory of Aberrations, Oxford University Clarendon Press, 1950; and R. R. Shannon, The Art and Science of Lens Design, Cambridge University Press, 1997.
The above identified references, patent applications, and provisional patent applications are hereby incorporated in their entirety herein by reference.
The present invention shows the way to use combinations of refractive and reflective lens elements to produce the required lens designs.
It is an object of the invention to produce a diffraction limited optical lens having a high numerical aperture for wavelengths where the number of optical materials for refractive lens elements is small. It is a further object of the invention to produce a diffraction limited optical lens having a high numerical aperture having fewer lens elements at less cost than a refractive lens having equivalent parameters. It is a further object of the invention to produce a diffraction limited optical lens having a high numerical aperture which includes refractive elements and reflective elements with curved reflective surfaces. It is a further object of the invention to produce a diffraction limited optical lens having a high numerical aperture having two reflective elements, each of which receives light from an object plane symmetrically with respect to an optical axis normal to the reflective surface.
The present invention is a system, apparatus and method to use two focusing mirrors in a high numerical aperture, non-ring, lens design. The optical axis of each mirror are coplanar and intersect at the position of a turning mirror having a normal to the surface of the turning mirror which bisects the angle formed by the two optical axis.